Conventional CPMs (Channel Performance Monitors), or OPM (Optical performance Monitors) as they are sometimes called, are used to measure optical power in DWDM (Dense Wavelength-Division-Multiplexing) optical signals. Monitoring of performance is becoming more important, and some CPMs also measure signal power and are used for measuring OSNR (Optical Signal-to-Noise Ratio) in DWDM optical systems. However, such devices suffer from their inability to measure OSNR accurately when the OSNR is greater than about 15 dB. In particular, to determine OSNR in a DWDM optical signal these devices measure signal power at channel wavelengths and a noise level at wavelengths between the channel wavelengths. However, the noise level measured includes crosstalk from adjacent channels. Furthermore, for purposes of monitoring, conventional systems make use of a 1% optical tap to obtain 1% of the power of the DWDM optical signal. This means that very little noise from the DWDM optical signal is tapped for purposes of measurements. If the noise tapped from the DWDM optical signal is below the noise floor of the monitor, it will not be possible to obtain an accurate measurement.
One improvement is to replace the 1% optical tap with a 2% optical tap to increase the power to the CPM by 3 dB. However, tapping a larger percentage of the power of the DWDM optical signal for measurements results in an increase in optical-system loss during optical performance monitoring. Furthermore, crosstalk related errors are not addressed.
By way of background, a conventional arrangement used for measuring OSNR will now be described with reference to FIGS. 1 and 2. In FIG. 1, a 1% optical tap 110 is coupled to an optical amplifier 120 and to a CPM 130. An optical signal 140 is input into the 1% optical tap 110. The 1% optical tap 110 couples 99% of the power of the optical signal 140 to the optical amplifier 120 for amplification. In particular, 99% of the power over an entire wavelength spectrum of the optical signal 140 is coupled to the optical amplifier 120 by way of output optical signal 150. The 1% optical tap 110 also couples 1% of the power of the optical signal 140 to the CPM 130 for measuring OSNR. In particular, 1% of the power over the entire wavelength spectrum of the optical signal 140 is coupled to the CPM 130 by way of output optical signal 160.
The accuracy in the noise measurement is compromised when the output optical signal 160 contains a component of noise from the optical signal 140, which has a power that is less than or close to the noise floor of the CPM 130. This, in turn, compromises the accuracy in determining the OSNR. For example, in FIG. 2 a curve 210 represents the power of the optical signal 160 plotted as a function of wavelength, and a curve 220 represents the noise floor plotted as a function of wavelength. The curve 210 shows a noise level of approximately −50 dBm at wavelengths between channel wavelengths 240 of the optical signal 160. The curve 220 shows a higher noise floor of approximately −46 dBm due to the CPM 130. Since the noise floor is higher than the actual noise of the output optical signal 160, existing CPMs cannot accurately measure the OSNR. The problem is exacerbated by the crosstalk problem. Conventional CPMs use a tunable filter that captures energy in a range of wavelengths. An example filter response is indicated at 245 shown tuned to a noise wavelength. Almost all energy gets captured at the centre wavelength, this being tunable. However it can be seen that when the frequency is selected to be between two channel wavelengths as shown, a significant part of the adjacent channel power is also captured making the noise power readings highly inaccurate.